1. Field of the Invention
The present invention relates to a multi-bored flat tube for use in a heat-exchanger and, more particulary, to a multi-bored flat tube made of a metal such as an aluminum for use in a condenser for an air conditioner. The present invention further relates to a heat exchanger including the multi-bored flat tubes.
2. Description of the Related Art
FIGS. 14(A)-(C) show cross-sectional views of a conventional multi-bored flat tube of this kind. The multi-bored flat tube 51 is made by extruding an aluminum. The tube 51 has a peripheral wall 52 having an elongated circular cross-sectional shape and a plurality of divisional wall 53, 53a connecting flat wall portions 52a, 52a of the peripheral wall 52. The divisional walls 53 divide an inside space of the tube 51 to form a plurality of unit passages 54, 55 arranged in a lateral direction of the tube 51. Each divisional wall 53, 53a has a constant thickness along the height thereof so that a contact area with the heat exchanging medium can be enlarged, thereby enhancing the heat exchanging performance of the tube 51. The tube 51 includes outermost unit passages 54, 54 and intermediate unit passages 55 located between the outermost unit passages 54, 54. Each intermediate passage 55 has a rectangular cross-sectional shape, and each outermost unit passage 54 has a semi-circular cross-sectional shape at a lateral outside portion and a rectangular cross-sectional shape at lateral inside portion. Further, each portion of the tube 51, i.e., the peripheral wall 52 and the divisional walls 53, 53a, are formed to be as thin as possible for the purpose of lightening the weight of the tube 51.
Japanese unexamined Utility Model Publication No. S60-196181 and Japanese examined Utility Model Publication No. H3-45034 disclose a tube having unit passages with inner fins formed on an inner surface of each unit passage to enlarge a contact area with the heat exchanging medium for the purpose of enhancing the heat exchanging performance. For example, as shown in FIGS. 15A and 15B, a tube 52 has a plurality of inner fins 62 formed on the inner surface of the unit passages 54, 55 surrounded by the peripheral wall 52 and the divisional walls 53, 53a. Each fin 62 has a triangular cross-sectional shape and extends in the longitudinal direction of the tube 61.
Japanese unexamined Patent Publication No. H5-215482 discloses another type of heat exchanging multi-bored flat tube. The tube has a plurality of unit passages each having a round cross-sectional shape for the purpose of equalizing the flow speed of the heat exchanging medium and lowering the flow resistance of the heat exchanging medium in each unit passage. FIGS. 14 and 15, the reference numeral 57 denotes a corrugate fin interposed between the adjacent tubes 61.
In a heat exchanger including the above-mentioned flat tubes 51, 61, a stress caused by an inner pressure of the heat exchanging medium passing through the tube is concentrated on connecting portions between the divisional wall 53, 53a and the peripheral wall 52. The lateral middle portion of the tube 51, 61 can withstand such a stress because the flat wall portions 52a of the peripheral wall 52 are supported and reinforced by the corrugate fins 57, 57. However, the lateral end portions of the tube 51, 61 are not strong enough to withstand such a stress because reinforcing effects obtained by the corrugate fins 57, 57 are not enough. Therefore, such a stress tends to be concentrated on the connecting portions between the outermost dividing wall 53a and the peripheral wall 52 to cause a breakage.
Further, as shown in FIGS. 14B and 14C, the above-mentioned tubes used in a condenser mounted in an automobile may sometimes be damaged and cause leakage of the heat exchanging medium when a stone, or the like, hits the tube while the automobile is moving.
The above-mentioned problems may be solved by thickening the dividing wall portion 53, 53a and the peripheral wall 52. However, this causes an increase in the tube weight, resulting in an increase in the heat exchanger weight.
In a tube having a plurality of unit passages each having a perfect circular cross-sectional shape, a flow resistance of heat exchanging medium passing through the unit passage can be decreased and the pressure resistance can be improved. However, upper and lower portions of each dividing wall are thicker than the middle portion thereof, which requires larger amount of material for forming the tube, thereby increasing the manufacturing costs. Further, within a limited tube thickness, a heat transferring area of the circular cross-sectional unit passage is smaller than that of the rectangular cross-sectional unit passage, resulting in a lower heat exchanging efficiency.